2026-04-04
Aluminum marks under hard jaws at 30 MPa of contact pressure. Titanium doesn't mark until 200 MPa. Clamping titanium with the same strategy as aluminum wastes 85% of the available clamping force — and the part moves.
Each material property affects a different fixturing decision. Yield strength determines the clamping force ceiling — exceed it and the jaw marks the part. CTE (coefficient of thermal expansion) determines clearance budget — a high-CTE material grows more per degree of temperature change, requiring looser clearance. Thermal conductivity determines whether cutting heat accumulates at the fixture interface or dissipates quickly.
The marking threshold is approximately 10% of yield strength on finish surfaces. Below that, jaw contact leaves no visible witness marks.
Material Yield (MPa) CTE (um/m/C) Conductivity (W/m-K) Marking ~(MPa)
─────────────────────────────────────────────────────────────────────────────────────
6061-T6 Al 276 23.6 167 ~28
1018 Steel 370 11.7 51.9 ~37
4140 Steel 655 11.2 42.6 ~66
303 SS 240 17.2 16.2 ~24
Ti-6Al-4V 880 8.6 6.7 ~88
PEEK 100 47 0.25 ~10
Delrin (POM) 70 110 0.37 ~7Yield 276 MPa. CTE 23.6 um/m/C. Thermal conductivity 167 W/m-K.
Soft, marks easily. Hard jaw serrations leave witness marks on finish surfaces at moderate clamping force. Use aluminum soft jaws — softer than or equal hardness to the part — to eliminate jaw marks entirely. The jaw deforms before the part does.
High CTE means the part grows with temperature. A 100 mm aluminum part gains 0.024 mm per degree C of temperature rise. Over a 10°C rise from cutting heat, that is 0.24 mm of growth — more than a typical 0.15 mm clearance. Generous clearance (0.15–0.20 mm) accommodates this. Aluminum conducts heat well, so the fixture warms up too, but it also cools fast with high-volume coolant.
Recommended setup: aluminum soft jaws, 0.15 mm clearance, flood coolant. Hard jaws acceptable for roughing operations where surface finish does not matter.
Yield 370–655 MPa. CTE 11–17 um/m/C. Thermal conductivity 16–52 W/m-K.
Moderate thermal expansion — standard clearances (0.15–0.20 mm) work. Tolerates harder clamping without marking. Serrated hard jaws are acceptable for roughing operations across all three grades.
1018 mild steel. Low yield (370 MPa) but high enough for most clamping. Standard setup. No special considerations beyond normal clearance.
4140 pre-hardened. Yield 655 MPa. Can use tighter clearances (lower CTE at 11.2 um/m/C means less thermal growth) and higher clamping force (higher yield ceiling). Good candidate for aggressive roughing setups where rigidity matters more than surface finish.
303 stainless. Yield only 240 MPa — lower than 6061-T6 aluminum. Gummy material that smears under concentrated pressure. Use soft jaws for finish surfaces. The low thermal conductivity (16.2 W/m-K) means heat accumulates at the cut zone more than with carbon steels. Moderate coolant flow helps.
Yield 880 MPa. CTE 8.6 um/m/C. Thermal conductivity 6.7 W/m-K.
Three consequences for fixturing:
Massive clamping force ceiling. Titanium can take clamping pressures that would destroy aluminum. The marking threshold is approximately 88 MPa — three times what aluminum tolerates. Use it. Titanium's high strength means cutting forces are high, and the fixture must resist them. Under-clamping titanium causes part movement during heavy roughing passes. Serrated hard jaws are appropriate.
Tight clearances are safe. CTE of 8.6 um/m/C is one-third of aluminum. A 100 mm titanium part gains only 0.009 mm per degree C — less than a tenth of a millimeter over a 10°C rise. Clearances of 0.10–0.15 mm are safe and improve rigidity by reducing part movement in the pocket.
Terrible thermal conductivity. At 6.7 W/m-K, titanium conducts heat 25x worse than aluminum. Cutting heat stays at the cut zone. The part gets hot locally while the fixture stays cool. This thermal gradient causes localized expansion that differs from the uniform growth model. Flood coolant is mandatory — not mist. Direct the flood at the cut zone to remove heat before it conducts into the fixture interface.
Recommended setup: serrated hard jaws, supplemental side support where geometry allows, 0.10–0.15 mm clearance, flood coolant, climb milling to reduce cutting forces.
Yield 70–100 MPa. CTE 47–110 um/m/C. Thermal conductivity 0.25–0.37 W/m-K.
Three problems converge:
Very low yield. PEEK yields at 100 MPa, Delrin at 70 MPa. The marking threshold is 7–10 MPa — a fraction of what metals tolerate. Steel jaw serrations leave permanent deformation, not just surface marks. Even smooth jaws can deform thin-walled plastic parts under moderate clamping force.
Extreme CTE. Delrin at 110 um/m/C grows 10x more than aluminum per degree. A 100 mm Delrin part gains 0.11 mm per degree C of temperature rise. Over a 10°C rise, that is 1.1 mm — enough to jam the part in a tight pocket or cause it to lose contact with the pocket walls as it cools. Clearance must account for this: 0.20–0.30 mm minimum, potentially more for large parts or operations that generate significant heat.
Creep under sustained load. Plastics deform slowly under constant stress. A part clamped at moderate pressure for an extended operation will slowly change shape in the fixture. The longer the cycle time, the worse this gets. Minimize clamping force and cycle time. If the operation requires extended machining, re-seat the part between operations.
Recommended setup: vacuum fixturing where the geometry allows (flat reference surface required). If using soft jaws: maximum contact area to distribute clamping force, minimum clamping pressure, aluminum jaws — not steel. Steel jaws are harder than the part by a factor of 5–10 and leave permanent marks.
Composite materials — carbon fiber, fiberglass, Kevlar layups — are anisotropic. Their mechanical properties depend on fiber orientation. Clamping perpendicular to fibers causes delamination. Clamping parallel to fibers compresses the matrix without engaging the reinforcement.
Composites require an entirely different fixturing approach: vacuum on flat tool plates, adhesive bonding, or conformal fixtures that distribute load across the surface. The material-property-to-clamping-strategy mapping in this article assumes isotropic materials and does not apply to fiber-reinforced composites.
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